B&N, Sony, and iBooks all use ePubs, as do others, which are essentially the same as far as the ePub structure goes. However, they each use their own unique DRM (digital rights management). Therefore you cannot buy an ePub from one of those stores and easily view it on the device from a different company unless two of the companies use Adobe ADE.

This is a major pain to those of us who read and would like to be able to buy an ePub from any store and then read it on any device that supports ePubs. However, from a publisher's perspective they would develop the ePub then simply attach the appropriate DRM for each store via the development tools they use.

BTW, ePubs are basically a set of HTML files with a CSS stylesheet that are compressed into a single file. They use a sub-set of basic HTML. If you use basic HTML and clean, standard HTML your books will most likely look okay. I would avoid using tables as they don't work well on small eBook readers. I would also avoid using too many styles. In other words, do not call out a specific font, font type, font size, text color, overall margin sizes, etc. It is best to let the reader (person reading) set these styles to their own tastes using their ePub reader device.